bug#16475: 24.3.50; enhancement request: remove vertical scroll bar automatically when not needed

[Keith David Bershatsky]

If and when feature request 22404 ever comes to fruition (to accurately 
calculate window-start/window-end every command loop under all circumstances), 
the following is a minor mode that demonstrates how to add/remove scroll bars 
depending upon the length of the buffer.  It can undoubtedly use some 
improvements, but I am already successfully using it for my own personal setup 
with the latest draft/patch for the `window-start-end-hook`.

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=22404

[NOTE:  Eli has indicated that a `post-redisplay-hook' (which does not 
introduce user Lisp into the redisplay mix) would be a better alternative to 
the proposed `window-start-end-hook`, and I completely respect Eli's opinion in 
that regard.  However, I wanted to get something working for my own personal 
use and I am enjoying familiarizing myself with some basic C language coding 
relating to feature 22404.]

Thanks,

Keith

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;;; sb-mode

;; https://github.com/kentaro/auto-save-buffers-enhanced
;; `sb-regexp-match-p` function modified by @sds on stackoverflow
;; http://stackoverflow.com/a/20343715/2112489
(defun sb-regexp-match-p (regexps string)
  (and string
       (catch 'matched
         (let ((inhibit-changing-match-data t)) ; small optimization
           (dolist (regexp regexps)
             (when (string-match regexp string)
               (throw 'matched t)))))))

(defgroup sb nil
  "A group for the lawlist scroll-bar."
  :tag "Scroll Bar"
  :group 'convenience)

(defvar regexp-always-scroll-bar '("\\.yes" "\\*Scroll-Bar\\*")
  "Regexp matching buffer names that will always have scroll bars.")

(defvar regexp-never-scroll-bar '("\\.off" "\\.not" "\\*Calendar\\*")
  "Regexp matching buffer names that will never have scroll bars.")

(defun lawlist-scroll-bar-fn (win start end pbol-start peol-end fully-p)
  (when
      (and
        (window-live-p win)
        (not (minibufferp)))
    (let* (
        (buffer-size (buffer-size))
        (point-min (point-min))
        (point-max (point-max))
        (window-scroll-bars (window-scroll-bars))
        (buffer-name (buffer-name)))
      (cond
        ;; not regexp matches | not narrow-to-region
        ((and
            (not (sb-regexp-match-p regexp-always-scroll-bar buffer-name))
            (not (sb-regexp-match-p regexp-never-scroll-bar buffer-name))
            (equal (- point-max point-min) buffer-size))
          (cond
            ;; Lines of text are less-than or equal-to window height,
            ;; and scroll bars are present (which need to be removed).
            ((and
                (<= (- point-max point-min) (- end start))
                (or
                  (equal window-scroll-bars '(15 2 right nil))  ;; GNU Emacs 
24.5.1 of 2015-04-10 on builder10-6.porkrind.org
                  (equal window-scroll-bars '(15 2 right 0 0 nil)) ;; 10/01/2014
                  (equal window-scroll-bars '(15 2 right nil 0 nil)) )) ;; 
11/19/2014
              (set-window-scroll-bars win 0 'right nil))
            ;; Lines of text are greater-than window height, and
            ;; scroll bars are not present (which need to be added).
            ((and
                (> (- point-max point-min) (- end start))
                (or
                  (equal window-scroll-bars '(0 0 nil nil)) ;; GNU Emacs 24.5.1 
of 2015-04-10 on builder10-6.porkrind.org
                  (equal window-scroll-bars '(0 0 t nil)) ;; GNU Emacs 24.5.1 
of 2015-04-10 on builder10-6.porkrind.org
                  (equal window-scroll-bars '(0 0 t 0 0 t)) ;; windows-nt 
(default)
                  (equal window-scroll-bars '(nil 0 t nil 0 nil)) ;; darwin 
(default) ;; 11/19/2014
                  (equal window-scroll-bars '(0 0 t 0 0 nil)) ;; darwin 
(default) ;; 10/01/2014
                  (equal window-scroll-bars '(0 0 nil nil 0 nil)) ;; 11/19/2014 
(after scroll bars are removed)
                  (equal window-scroll-bars '(0 0 nil 0 0 nil)))) ;; windows-nt 
/ darwin
              (set-window-scroll-bars win 15 'right nil))))
        ;; Narrow-to-region is active, and scroll bars are present
        ;; (which need to be removed).
        ((and
            (buffer-narrowed-p)
            (or
              (equal window-scroll-bars '(15 2 right nil))  ;; GNU Emacs 24.5.1 
of 2015-04-10 on builder10-6.porkrind.org
              (equal window-scroll-bars '(15 2 right 0 0 nil)) ;; 10/01/2014
              (equal window-scroll-bars '(15 2 right nil 0 nil)) )) ;; 
11/19/2014
          (set-window-scroll-bars win 0 'right nil))
        ;; not narrow-to-region | regexp always scroll-bars
        ((and
            (equal (- point-max point-min) buffer-size)
            (eq major-mode 'calendar-mode)
            (sb-regexp-match-p regexp-always-scroll-bar buffer-name))
          (set-window-scroll-bars win 15 'right nil))
        ;; not narrow-to-region | regexp never scroll-bars
        ((and
            (equal (- point-max point-min) buffer-size)
            (sb-regexp-match-p regexp-never-scroll-bar buffer-name))
          (set-window-scroll-bars win 0 'right nil))))))

(defvar sb-mode-display-hook nil
"Hook used primarily for `lawlist-scroll-bar-fn'.")

(define-minor-mode sb-mode
"This is a minor-mode for lawlist-scroll-bar."
  :init-value nil
  :lighter " SB"
  :keymap nil
  :global nil
  :group 'sb
  (cond
    (sb-mode
      (add-hook 'window-start-end-hook 'lawlist-scroll-bar-fn nil t)
      (when (called-interactively-p 'any)
        (message "Globally turned ON `sb-mode`.")))
    (t
      (remove-hook 'window-start-end-hook 'lawlist-scroll-bar-fn t)
      (set-window-scroll-bars (selected-window) 0 'right nil)
      (when (called-interactively-p 'any)
        (message "Globally turned OFF `sb-mode`.") ))))

(provide 'lawlist-sb)